Electric coupling system



Jan. 1, 1946. A AN E 2,392,170

ELECTRIC COUPLING SYSTEM Filed April 10, 1944 AMPLIFIER AMPLIFIER AMPL/F/EH AND SIGN/1L HEPHODUCER Fig. 2.

Inventor: Arthur G. Ma he,

b mwy l 8 His Attorney.

Patented Jan. 1, 1946 ELECTRIC COUPLING. SYSTEM Arthur G. Manke, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application; April 10, 1944, Serial; No. 530,346

8 Claims.

invention relates to electric couplingsystoms. and, more particularly; to. beat frequency oscillator transformers of: the type. utilized. in radio: reception systems.

In radiotelegraph reception systems, it is; customary to render-- an; unmodulated' carrier wave audible, bybeating with the received carrier, or

with. a, carrier of suitably reduced intermediate frequency, local oscillations of nearly equal fre quency generated in a beat frequency oscillator. One: known manner of injecting the local oscillations from the beat frequency oscillator into the carrier wave channel is by couplin to the grid circuit of: an intermediate frequenc amplifier stage preceding the second detector. For this purpose th intermediate frequency transformer may: be provided with a third winding connected to the beat frequency oscillator. In any such arrangement, it is, desirable. that the oscillations in the transformer grid coil, from the, beat frequency oscillator be at least equal in amplitude to the oscillations of carrier, frequency in the same; coil in, order fully to load the output stages ofthe receiver. On theother hand, care must be exercised, that the amplitude of oscillations from the: beat frequercyoscillatoris not so great as to opcratethe automatic volume control circuits assooiated. with subsequentstages of the receiver.

It has beenfound that-theelements of thebeat frequency. transformer must, be rigidly mounted in fixed relation during manufacture to ensurefrequency stability and. reliability of operation. Furthermore, since it is necessary tomaintain maximum suppression of oscillations at the frequency of the-beat frequency oscillator in other parts of the receiver, it is necessary: that: the oscillator coil. and the grid coil: remain tuned accurately to the frequency of the beat frequency oscillaton. It is, therefore, impractical to con-.

trol -the amplitude of local oscillations; in the grid coil; either byvarying; the coil; tuningv or the coil coupling. Accordingly, it-is ageneralr o ject of my inVn-.-- tion. to prVide,a new and novel coupling system andmothod for beating togetherelectric, oscillationsgof'different frequencies.

Another object of my invention;istheprovision ofa new and novel method and means for adjustg, t e; amplit de of electric; oscillations. introduced into. a circuit, from one source without appreciably affecting oscillations of nearly equal frequency induced" in the samecircuit from. another sources It is a more. specific. object of, my invention. to. provide means for; adjusting the amplitude of oscillations-induced in the grid coil of. a beat frequency transformer from the beat frequency oscillator without changing the tuning, coupling. or

spacing of either the grid coil or the oscillatorcoil.

My invention will be more fully understood and its objects and advantages further appreciated by referring now to the following detailed specification. taken in conjunction with the accompany-- ing drawing in which Fig. 1 is a schematic circuit diagram of a radio receiving apparatus embodying' my invention; and Fig, 2 is a side elevation, partly in section, of apractical embodiment ofthe combined beat frequency oscillator and intermediate frequency transformer of the type illustrated schematically at-Fig. l.

Referringnow to the drawing; and particularly to Fig. l, I have shown a radiotelegraph receiving apparatus comprising an antenna l9 coupled to a signal channel comprising aplurality of stages. of amplification H and I2 and a detector l3. Preferably, the receiving apparatus is of. the $11-- perheterodyne type and comprises also a suitable number of stages of radio frequency amplification, a.local oscillator and a mixer for converting the. carrier wave to an intermediate frequency of predetermined fixed value, and a suitable number of stages of intermediate frequency amplification preceding the stage II. These preceding stages of the receiving apparatus are shown in block form and identified by the reference numeral M. It willtherefore be understood that that portion of the signal channel represented by the reference numeral [4 supplies to an intermediate frequency transformer 15 an unmodulated carrier Wave of predetermined fixed intermediate frequency.

Throughthe transformer l5 theintermediate frequency carrier wave is impressed upon a control. grid I6 of an electron discharge device I! in. the amplifyin stage H. The discharge device-l!- comprises a cathode Ila, a screen electrode [1b and a plate He. The cathodel'la is connected to ground through, a grid leak. resistor He and. bypass-condenser, l'ld. The'plate I 10- is connected to a resonant output circuit comprising a transformer win-ding 18 of variable inductarce and a condenser !9. connected in parallel circuit relation. Thetransformer winding I8 is the primary Winding, or plate coil, of a combined beat frequency. oscillator and intermediate frequency transformer 20. The transformer 20 is also providedwith avariable inductancesecondary windingor grid. coil. 2| closely coupled to thewinding l8.v and tuned to resonance. at. adesired inter.- mediate frequency. by a capacitor 22 and connected between the cathode 23 and the control grid 24 of an electron discharge device 25 in the amplifying stage i2. The cathode 23 is connected to ground through a grid leak resistor 23a and a by-pass condenser 23b. The discharge device 25 includes also a plate 26 connected to an intermediate frequency coupling transformer 21 having primary and secondary windings 28 and 29 tuned to resonance at the intermediate frequency by parallel-connected capacitors 30 and 3|, respectively. The discharge device 25 is preferably provided also with a screen electrode 26a connected to a suitable source of positive 'unidirectional potential, as shown.

The secondary winding 29 of the intermediate frequency transformer 21 supplies oscillations to the diode detector circuit I3. The diode detector comprises the anode 32 and cathode 33 of a double diode electric discharge device 34 connected in series circuit relation with a load resistor 35 and u a decoupling resistor 36 across the terminals of the transformer winding 29. The resistors 35 and 36 are shunted by a capacitor 31' for bypassing carrier and other high frequency oscillations. The cathode 33 of the diode 34 is grounded for high frequency potentials through a condenser 33 and is maintained at a positive unidirectional potential with respect to ground by a cathode resistor 39. The resistor39 is connected between the cathode 33 and ground and across a suitable source of unidirectional potential 40 through a resistor 40a. Audio frequency voltages appearing across the'load resistor 35 are supplied to audio frequency amplifiers and thence to a suitable signal reproducing device, such as a loud speaker, head phones or the like, all indicated in block form upon the diagram by the reference numeral 4|.

To provide automatic volume control, the gain of one or more preceding stages of the receiver is'co'ntrolled by a bias potential derived from the output of the intermediate frequency transformer 21 through a diode rectifier comprising a second anode 42 and the cathode 33 of the electric discharge device 34. The anode 42 is connected to one terminal of the transformer 21 through a coupling condenser 43 and a load resistor 44 is connected between the anode 42 and ground. The negative bias potential appearing across resistor 44 is supplied through a suitable decouplin resistor 45 and filter condenser 45a to control the gain of a preceding stage of the receiver.

It will be noted that the cathode biasing resistor 39 associated with the double diode discharge device 34 has no appreciable effect upon the audio frequency detection circuit, but provides for fdelayed automatic volume control operation. The delayed operation of the volume control circuit results from the fact that the automatic volume control section of the double diode 34 is not rendered conductive until the carrier wave exceeds a predetermined amplitude suiiicient to raise the potential of the anode 42 above the positive bias potential of the cathode 33. It is only when the anode 42 of the double v diode 34 is conducting that a volume control bias is developed across the resistor 44, By this arrangement operation of the volume control circuit by the beat frequency oscillations is prevented. V

Referring now again to the beat frequency transformer, it may be noted that the unmodulated carrier wave appearing in the trans- V former primary winding 8 is modulated by beat ing with'it electric oscillations of a nearly equal frequency supplied by a beat frequency oscillator 5|]. The beat note is then rendered audible by detection in the circuit l3. In order to modulate the received carrier wave the transformer 20 is provided with a tertiary winding 5| tuned to resonance at a desired frequency by means of a variable capacitor 52 and connected through capacitors 53 and 54, respectively, to the plate 55 and control electrode 56, respectively, of an electron discharge device 51. The winding 5| is closely coupled to the winding 2| and loosely coupled to the winding iii. The cathode 58 of the discharge device 51 is connected directly to an intermediate point of the coil 5| and through a grid biasing resistor 59 to the control grid 55. The anode 55 of the discharge device 51 is connected through a decoupling resistor 50 to a suitable source of positive potential indicated as 18+. It will now be evident that the electron discharge device 51 and the tuned circuit comprisingthe transformer Winding 5| and tuning condenser 52 constitute an oscillator of the well-known Hartley type and function to generate in the coil 5| electric oscillations of a desired frequency.

It will, of course, be understood that I have shown the particular oscillator 59 by way of illustration only, and that my invention is not limited thereto but contemplates any suitable source of local oscillations of desired frequency coupled to the grid coil 2|. If desired, the oscillator triode 51 and the double diode 34 may be built within a single discharge envelope. In such case, the cathodes 33 and 58 shown on the drawing would be the single cathode of the combination tube, so that the oscillator cathode, as well as the diode cathode, would be biased positively wit respect to ground by the resistor 39.

Preferably, the beat frequency transformer windings l8, 2 and 5| and theirassociated tuning condensers I9, 22, and 52 are physically po-- tional view of such a transformer mounted with-.

in a metallic casing 53 and comprising a suitable fixed support, such as a tube 64 of insulating ma-' terial, carrying the plate coil i8 and grid coil 2| and a similar support 65 carrying the oscillator coil 5|. The insulating supporting tubes may, by way of example, be mounted in fixed insulat; ing supporting plates 64a and 65a. Adjustable iron core members 66 and 51 for the coils l8 and 2|, respectively, are disposed in the hollow ends of the tube 54. The iron cores 65 and 61 may be arranged threadingly to engage the ends of the casings 53 so that their positions within the coils l8 and 2| may be adjusted to control the coil inductance. It will be understood that, While the tuning condensers I9, 22, and 52 are not shown at Fig. 2, they are preferably mounted also within the casing 63 to provide optimum shielding of all circuits associated with the beat frequency oscillator. 50 from other parts of the receiving apparatus. By this positioning of the coils upon" the separate supports 54 and 65, the coils I8 and 5| are both closely coupled to the coil 2| and loosely coupled to each other. 7

To ensure reliability and frequency stability in operation, it is necessary that the windings l3, 2| and 5| and the supporting members 64 and B5 of the beat frequency transformer. shown at Fig. 2 be rigidly'and immovably mounted within the casing 63. In manufacture, the spacing of.- the supporting members and the spacing of the ducedin the grid coil. This induced voltage must be kept within certain limits because, while it is desirable to obtain 100 per cent modulation of the received carrier waves so that the output stages of the receiver will be fullyloa-ded by the signal wave, it is undesirable that the voltage induced in the grid coil 21 from the beat frequency oscillator coil be so large that the automatic volume control circuit is brought into operation in the presence of a carrier wave of desired amplitude. It-is, therefore, desirable to provide means for controlling the amplitude of the beat frequency oscillations induced in the grid coil 2| so that these oscillations will not operate the automatic limiting circuit but are slightly greater in amplitude than the oscillations of carrier frequency induced in the coil 2| from the coil I8. While it is possible tocontrol the amplitude of oscillations in the grid coil 2| from the beat frequency oscillator 50 by detuning the resonant circuit 2| 22, this is undesirable because maximum suppression of undesired responses from the beat frequency oscillator 50 is attained when the resonant circuit of the oscil-- lator and the resonant circuit 2|, 22 are both tuned to substantially the frequency of theoscillations supplied by the oscillator 50. Furthermore, as previously pointed out, the amplitude of oscillations induced in the coil 2| from the coil 5| cannot be controlled by varying the coupling of the coils by reason of the fact that the coils are fixedly mounted within the transformer casing 63.

Preferably, the grid coil tank circuit 2|, 22 is tuned approximately to the second harmonic of the fundamental frequency of the oscillator 59, so that the beat frequency oscillations mixed with the carrier wave in the transformer 20 have doublethe fundamental frequency of the local oscillations generated. This is desirable in order to prevent absorption of the intermediate frequency signals due to the beat frequency oscillator tank circuit 5|, 52. Furthermore, operation at a harmonic of the beat frequency oscillatorreduces the tendency of the oscillator 50 topull into step with the intermediate frequency oscillations. The beat frequency oscillations are injected into the signal channel at a point preceding the amplifier 25, as into the transformer 20, so that the intermediate frequency amplifier 25 may be used to amplify the beat frequency oscillations along with the intermediate frequency carrier. In this manner a beat frequency oscillator of relatively small power may be used.

According to my invention, the amplitude of oscillations coupled into the grid coil 2| from the beat frequency oscillator 50 is controlled by a means physically separate from both the tuned grid circuit 2|, 22 and the tuned beat frequency oscillator circuit 5|, 52. For this purpose, I utilize the adjustable iron core 66 of the plate coil l8. By varying the tuning of the resonant circuit |8, |9 with respect to the resonant circuit 2|, 22, I find that the apparent impedance reflected back to the tuned grid circuit 2!, 22 from the tuned plate circuit l8, l9 may be varied in order very effectively to control the amplitude of oscillations induced in the coil 2| from the coil 5| without varying the tuning, coupling or spacing of the coils 2| and 5|. For this purpose, the

coils l8 and '21 are preferably coupled slightly over their critical value, so that slight de'tuning of the resonant plate circuit I8, is with respect to the resonant grid circuit 2|, 22 has little appreciable effect upon the amplitude of the carrier wave oscillations induced in the coil 2| from the coil f8.

It is contemplated that both the carrier wave supplied to the transformer winding H3 and the oscillations supplied from the oscillator 50 are of high frequency in relation to the low audio difference frequency appearing across the detector load resistor 35. By way of example, the intermediate carrier wave frequency may be kilocycles per secondwhile the second harmonic frequency supplied to the transformer 20 from the local beat frequency oscillator '50 may be 159 kilocycles per second. The detected audio frequency'is thus 1900 cycles per second. The oscillator fundamental frequency to which the coil 5| is tuned is 79.5 kilocycles per second. Preferably the grid coil 2| is tuned to the nominal intermediate frequency of 160 kilocycles per second. The tuning is sufiiciently close to the oscillator second harmonic at 159 kilocycles to respond strongly to the second harmonic without interference from other harmonics. The plate coil l8 may then be normally tuned to 160 kilocycles per second and subsequently slightly detuned with respect to the coil 2| if it is desired to increase the strength of the beat frequency signal. If it is desired to permit decrease of the beat frequency signal strength as well as an increase, thecoil |8 may be normally tuned slightly on either side of the 160 kilocycles to which the coil 2| is tuned. In this latter case the beat frequency signal is increased as the coil I8 is further detuned and decreased as the tuning of the coil It approaches the resonant frequencyof the coil 2|. It is also possible to tune the grid coil 2| to the oscillator frequency of 159 kilocycles per second or to some frequency between 159 kilocycle and the intermediate frequency of 160 kilocycles, While the plate coil I8 is normally tuned to 160 kilocycles. In this case also, the strength of the beat frequency signal may be controlled by bringing the frequency of the coil [8 toward or away from that of the coil 2| to control the reflected impedance.

In any case, in order to maintain a minimum of disturbance, it is desirable that the tuning of the plate coil l8 with respect to the grid coil 2| be carried out on the opposite side of the interm diate frequency from that to which the oscillator coil 5| is tuned. That is. if the coil 5| is tuned to 79.5 kilocycles per second, as in the above example, the coil 2| to 160 kilocycles per second, and the coil I 8 normally to .160 kilocycles per second, the resonant frequency of the coil l8 should be increased rather than decreased to increase the beat frequency signal. By way of illustration, it has been found possible to control the amplitude of the oscillations induced in the coil 2| from the oscillator coil 5| over a range of approximately 2:1 without appreciably affecting the transmission of oscillations of carrier wave frequency through the transformer.

While I have illustrated only one preferred embodiment of my invention by way of illustration, many modifications will occur to those skilled in the art, and I therefore wish to have it understood that I intend in the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

-What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an electric coupling system comprising a pair of resonant circuits in closely coupled inductive relation and a source of electric oscillations closely coupled to only one of said resonant circuits, the method of controlling the amplitude of oscillations from said source in said one resonant circuit which comprises varying the tuning of the other of said resonant circuits withrespect to said one resonant circuit.

2. In an electric coupling system comprising a pair of resonant circuits in closely coupled induc- 'tive relation and a source of electric oscillations of predetermined frequency closely coupled to only one of said resonant circuits, the method of modulating a carrier wave of another frequency which comprises impressing said carrier wave upon said one resonant circuit, coupling into said one resonant circuit oscillations from said source,

and varying the tuning of said other resonant circuit to control the amplitude of oscillations from said source in said one resonant circuit.

3. In an electric coupling system comprising a pair of resonant-circuits in closely coupled inductive relation tuned to substantially the same frequency and a third resonant circuit closely coupled to only one of said pair of resonant circuits and-tuned to the frequency of said one resonant circuit, the method of modulating electric oscillations of one high frequency with oscillations of a second high frequency which comprises inducing oscillations at said one frequency in said one resonant circuit from the other of said pair of resonant circuits, coupling oscillations at said second frequency from said third resonant circuit to said one resonant circuit, and varying the tuning of said other resonant circuit tocontrol the amplitude of oscillations of said second frequency in said one resonant circuit without appreciably affecting the amplitude of oscillations of said one frequency.

4.-In an electric coupling system comprising a pair of resonant circuits in closely coupled inductive relation with a third resonant circuit and looselyicoupled in inductive relation with each other, the method of modulating electric oscillations of one high frequency with oscillations of a second high frequency which comprises tuning said third resonant circuit and one of said pair of resonant circuits to a first high frequency, tuning the other of said pair of resonant circuits to a second high frequency, inducing a voltage at said one high frequency in said third resonant circuits from said one resonant circuit, inducing a second voltage at said second high frequency in said third resonant circuit from said other resonant circuit, and slightly detuning said one resonant circuit With respect to said third resonant circuit to control the amplitude of said second induced voltage without appreciably affecting the amplitude of said first induced voltage.

5. In an electric coupling system comprising a pair of loosely coupled resonant circuits each arranged to induce electric oscillations in a third resonant circuit closely coupled to each of said pair of circuits, the method of controlling the amplitude of oscillations induced in said third circuit from one of said pair of resonant circuits which comprises varying the tuning of the other of said pair of resonant circuits. 7

6. In combination, a signal channel for transmitting a carrier wave, said channel comprising a pair of resonant circuits in closely coupled inductive relation and tuned to different frequencies nearly equal to the frequency of said carrier wave, and additional means for coupling into only one of said resonant circuits electric oscillations at substantially the resonant frequency of said circuit, the amplitude ofsaid oscillations being determined by the relative detun ing of said pair of resonant circuits. 7

7. In combination, a signal channel arranged to transmit a carrier wave, said signal channel comprising a pair of resonant circuits in closely coupled inductive relation, a source of electric oscillations having a frequency different from the frequency of said carrier wave closely coupled to'only one of said resonant circuits, said one resonant circuit being tuned substantially to the frequency of oscillations from said source, and means for controlling the tuning of the-other of said resonant circuits with respect to said one resonant circuit to control the amplitude of oscillations from said source in said one resonant circuit.

8. A radio receiving apparatus comprising a signal channel arranged to transmit a high frequency carrier wave, said signal channel comprising a pair of resonant circuits in closely coupled inductive relation tuned to substantially the frequency of said carrier wave, a source of electric oscillations having a frequency substantially equal to the resonant frequency of one of said pair of tuned circuits and including a third resonant circuit closely coupled inductively to only said one resonant circuit, and means for varying the tuning of the other of said pair of resonant circuits to control the amplitude of oscillations induced in said one resonant circuit from said source.

ARTHUR G. MANKE. 

